Successful treatment of cancer by neutron or photon therapy necessitates a knowledge of the normal tissue sequelae associated with the cure of the tumor. This is expressed as the dose of photon irradiation which results in specific late effects (especially carcinogenesis) to the dose of neutron irradiation which results in the same specific effects (RBE values). The aim of this investigation is to obtain these values for sequelae in the brain, spinal cord, and lung, to investigate mechanisms of action of the two modalities in normal tissues and to test new protocols for neutron therapy in normal brain. Currently fast neutrons are being used in the United States, Europe and Japan in the treatment of cancer patients. New hospital-based facilities are under construction in the United States. Normal tissue tolerances have been approached, especially in the brain and spinal cord in the patient population which has been treated. Our data, which has been reported for three years, in large animals using a clinical fractionated schedule and total doses of neutrons and photon encompassing the clinical range have yielded RBE values of greater than 4 in the therapeutic dose range. The chronic effects of fast neutrons in rain, spinal cord, and lung of large animals using clinical exposure schedules and quantitative methodology subject to statistical analysis have not been reported elsewhere. This ongoing study will continue to evaluate sequelae utilizing functional and quantitative methods. Late effects on brain and spinal cord have been measured by sensory and visual evoked responses and will continue to be studied by quantitative CT scanning, onset of neurological deficits and quantitative histopathology. Late effects on lung tissue will be measured by radionuclide evaluation of pulmonary function (aerosol deposition and perfusion), quantitative histopathology and biochemistry, CT & radiographs. The onset, incidence and degree of malignancy will continue to be documented and evaluated by radiography, CT and histologic evaluation. RBE values are to be obtained for 2 new protocols for brain irradiation.